1. Field of the Invention
The present invention relates to a method, housing and a plant for the continuous/intermittent coating of objects by the passage of the objects through a bath a liquid coating product. The present invention applies in particular to the specific case of the galvanizing of metal objects with metal base or metal alloy products, but also to plants allowing to apply a liquid coating product of any other kind, such as certain resins or paints on certain metallic or non metallic objects.
2. Discussion of Background Information
In the field of metallurgy, plants for the continuous hot galvanizing of metal objects with zinc, aluminium or their alloys, in particular, are well known. A continuous galvanizing method using aluminium is described for example in French Patent FR-1 457 615 filed in the name of "Colorado Fuel and Iron Corporation", while continuous galvanizing with zinc and its alloys is described in French Patent FR-2 323 772 filed, in the name of Delot. In these two documents, it is proposed to improve the quality of the zinc or aluminium base anti-corrosion coating carried out on an elongated metal object, such as a concrete wire, in observing a common elementary principle concerning the intermetallic layer which developes in contact with the object's surface and the coating product. This layer is necessarily thin so as to avoid the risk of reducing the resistance of the superficial protective coating, as it is well established that a thick intermetallic layer tends to crack and come apart from the surface of the object it is supposed to protect.
This obtaining 8 of a thin intermetallic layer requires a very short intimate contact between a metallic object, which should be perfectly pickled and cleared of all its oxides, and a galvanizing bath at a temperature close to or slightly higher than that of the object, the bath also being perfectly free from any contact with an oxidizing agent (atmospheric air, floating matte composing a germ of oxides).
To reach this result, the techniques proposed in the two above mentioned patents are identical in that all the operations necessary for continuous galvanizing, i.e., the pickling and the heating of the object to be heated, then the quick intimate contact between the object and the bath in the housing, and perhaps the immediate cooling of the covered object (to stop the thermal diffusion causing the intermetallic layer to grow)-takes place under the controlled atmosphere of a neutral or reducing gas, maintained under pressure and at a temperature at suitable values (usually, at atmospheric pressure and at a temperature close to that of the object and the bath of zinc or molten aluminium). Another fundamental point common to both techniques consists in that the inlet and outlet holes to the galvanizing housing are aligned for the passage of the object to be covered, which makes possible continuous galvanizing. This method is far more advantageous than the competing galvanizing methods called "immersion coating", often applied to sheet metal. In these "immersion coating" methods, it is necessary to carry out an intermediate fluxing between the pickling and galvanizing, with the aim of this fluxing operation being to momentarily protect the cleaned surface of the object to be coated when it is exposed to the air, before immersion in the galvanizing bath.
Apart from the points they have in common, both of the above mentioned continuous galvanizing methods differ in particular in the elements used for pickling the object to be coated and for its heating, and especially in the elements used to seal the inlet and outlet holes to the galvanizing housing in which the molten aluminium or zinc bath is found. In this respect, it should be noted that it is more beneficial to use the zingage method described in French Patent FR-2 323 772 for the following reasons:
the pickling of the metal object to be coated is carried out mechanically (cold shot blasting), not chemically (reduction by hydrogen at a high temperature), which spares the inherent mechanical properties of the object generally made of steel, and for which there is a maximum temperature above which a change in its crystal structure occurs requiring annealing after galvanizing.
the heating, preferably by high frequency induction, is quicker and more profitable considering the energy balance of the plant, its control also being more accurate than heating by Joule effect. Moreover, in the case of certain steels having lost some of their mechanical qualities (especially elongation) due to cold drawing prior to their anti-corrosion treatment (namely concrete wire), an extremely short heating time combined with a galvanizing time, also very short, not only enables the avoiding of a structural modification of these steels, but also procures a rapid immersion of them, which allows the recuperation of their original mechanical properties prior to drawing.
In none of the previous methods is the tightness of the inlet and outlet holes to the galvanizing housing satisfactory, which causes leaks of the molten coating product outside the housing. These structural or accidental leaks should be recycled either through overflow holes especially provided in the wall of the housing or through one of the inlet or outlet holes of the housing. In both circumstances, to ensure the circulation of molten product from the melting furnace to the galvanizing housing or during the recycling of the same product between the latter and the melting furnace, the plants known for the setting up of the previous methods require the use of at least one pump. The continuous circulation of molten product within the plant causes an agitation in the melting furnace that might carry dross towards the galvanizing housing likely to cause obstructions within the circulation pump or in the various passages or conduits inside which the molten product circulates. Moreover even if there is no obstruction, this dross, floating on the galvanizing bath, could oxidize it and consequently alter the quality of the coating formed on the objects to be coated, as it clearly results from the continuous galvanizing principles shown in the methods described in the two above-mentioned methods.
In addition, it is important to note that the volume of the bath of molten coating product is always very important; however, as steel objects pass through the bath it becomes saturated in iron and an iron-zinc alloy is formed which is deposited at the bottom of the galvanizing housing in the form of mattes which are detrimental to the purity of the bath and consequently to the quality of the coating.
In fields other than metallurgy, identical problems occur in relation to the tightness of housings containing a liquid product for the coating of metallic or non-metallic objects, tightness defects then requiring a permanent recycling of the structural or accidental leaks occuring during the treatment for example, in the case of certain resins or paints, the hot or cold coating techniques are similar to those developed for metallization by hot galvanizing. Here again, the integrity of the liquid coating product should be preserved in the same way as a molten metal or metal alloy should be preserved from oxidation, whether it be in the housing where it is to be found in bath form or in the conduits for the recycling of the leaks of molten metal or metal alloy outside the housing.